1. Field of the Invention
The present invention relates to an optical scanning device that deflects light beams emitted from a plurality of beam-emitting units independently thereby scanning surfaces of different objects to be scanned, and that includes a light-receiving unit for receiving each of the deflected scanning beams at a predetermined position in the direction of deflection.
2. Description of the Related Art
Conventionally, optical scanning devices are widely used in digital image forming apparatuses. The optical scanning devices include a beam-emitting unit and a beam-deflecting unit. The beam-emitting unit includes a laser diode or the like and emits a light beam. The beam-deflecting unit is, for example, a polygon mirror that rotates or an oscillating mirror that oscillates back and forth. The beam-deflecting unit deflects the light beam emitted from the beam-emitting unit, applying the beam to a photoconductor, thereby optically scanning a surface of the photoconductor. The light beam is deflected so that the beam spot it forms on the surface of the photoconductor can move in a direction that is substantially at right angles to the direction in which the photoconductor moves. The direction in which the beam spot moves on the surface of the photoconductor is known as a main-scanning direction. If the photoconductor does not move at all, the light beam will only repeatedly scan the surface of the photoconductor, along the same line extending at right angles to the direction in which the photoconductor is moved. Since the surface of the photoconductor moves substantially at right angles to the main-scanning direction, the beam scans the photoconductor also in this direction. This direction (the direction in which the latent-image carrier moves) is known as sub-scanning direction. As the surface of the photoconductor is scanned in both the main-scanning direction and the subs-scanning direction, an electrostatic latent image is written on the photoconductor. The electrostatic latent image thus written is developed into a visible image by a developing unit that is incorporated in the image forming apparatus.
Optical scanning devices that perform optical scanning to accomplish so-called tandem image formation are known, as disclosed in, for example, Japanese Patent Application Laid-open No. 2003-98454 and Japanese Patent Application Laid-open No. 2004-271763. The tandem image formation is an image forming method in which electronic photographing is performed, forming visible images in parallel on the surfaces of photoconductors. The visible images are transferred from the photoconductors onto a transfer-recording medium and thereby superimposed one on another, thus providing a multi-color image. In the optical scanning devices disclosed in Japanese Patent Application Laid-open No. 2003-98454 and Japanese Patent Application Laid-open No. 2004-271763, light beams emitted from the beam-emitting units are deflected independently and scan different photoconductors, respectively. That is, the photoconductors are scanned in parallel. In this parallel scanning, a reflection mirror provided at one end of the scanning line reflects the light beams. A light-receiving unit, such as a light-receiving element, receives the light beam thus reflected and generates sync signals from the light beams. Timings of driving the beam-emitting units are determined from the sync signals. According to the timings thus determined, the timings of driving the beam-emitting units are adjusted, thereby minimizing the mutual displacement of the visible images on the transfer-recording medium.
Even if the timings of driving the beam-emitting units are so adjusted, the displacement of the visible images cannot be sufficiently reduced in some cases. The present inventors conducted an intensive research to find out why the displacement cannot be reduced sufficiently. The research has revealed the following. As shown in FIG. 15, a light-receiving sensor 200 includes a light-receiving surface 201 that is wider than the diameter of a laser beam L. The laser beam L, which is emitted from a photodiode or the like, is deflected by a beam-deflecting unit (not shown) such as a polygon mirror. As the laser beam L thus deflected moves over the light-receiving surface 201 in the direction of the arrow (scanning direction), it is detected by the light-receiving sensor 200.
The photosensitivity that the light-receiving sensor 200 has with respect to the laser beam L differs, depending on the incident angle of the laser beam L, even if the laser beam L remains unchanged in intensity. For example, the light-receiving sensor 200 exhibits relatively high photosensitivity to the laser beam L that is applied almost perpendicular to the light-receiving surface 201, or at an incident angle that is nearly equal to 0°, as shown in FIG. 16. The light-receiving sensor 200 such output-voltage characteristic as is shown in FIG. 17. As shown in FIG. 17, the greater the amount of light the light-receiving sensor 200 receives, the higher the output voltage of the light-receiving sensor 200. At the moment the light-receiving sensor 200 starts receiving the laser beam L moving in the scanning direction, only a part of the beam spot illuminates the light-receiving surface 201. Hence, the output voltage of the light-receiving sensor 200 gradually falls at a slope, as shown in FIG. 17, when the light-receiving sensor 200 starts receiving the laser beam L. Some time later the output voltage is stabilized at the minimum value. The output voltage falls relatively fast because the laser beam L is applied almost perpendicular to the light-receiving surface 201. At a time t1 when the voltage falls below a preset threshold value, the light-receiving sensor 200 is considered to have received the laser beam L. The output the light-receiving sensor 200 generates at this time is used as a sync signal in the control unit of the optical scanning device.
By contrast, the light-receiving sensor 200 has relatively low photosensitivity to the laser beam L applied to the light-receiving surface 201, much inclined thereto, as is shown in FIG. 18. In this case, when the light-receiving sensor 200 starts receiving the laser beam L, the output voltage falls as shown in FIG. 19, more slowly than in the case that the laser beam L is applied perpendicular to the light-receiving surface 201. A sync signal is therefore generated at a time t2, some time after the time t1. Consequently, sync signals will be detected at different times if the laser beams L for respective color images are applied at different incident angles. The phases of these sync signals cannot be detected with high accuracy, which makes it difficult to minimize the displacement of visible images.